Controllable oscillators (COs) generally are oscillators which output one or more output signals with a frequency, said frequency being determined by a control signal supplied to the controllable oscillator. Examples for controllable oscillators are voltage-controlled oscillators (VCOs) where the control signal is a voltage signal or digitally controlled oscillators (DCOs) and numerically controlled oscillators (NCOs) where the control signal is a digital signal.
Such controllable oscillators are for example controlled by a phase locked loop (PLL) to generate an output signal with a phase and/or frequency having a predetermined relationship with a phase and/or frequency of a reference signal supplied to the phase locked loop.
In PLLs generally the phase of the reference signal and a phase of a signal corresponding to or derived from an output signal of a controllable oscillator are aligned. For example in narrowband PLLs where the frequency range of the oscillator is comparatively small, it may take a relatively long time until phase alignment (also referred to as locking of the PLL) is reached.